Previous finds have established that competing species of trematodes within the snail may exercise a direct (predatory) or indirect (degenerative or histolytic) effect on one another. Various trematode combinations and host strains have been used to study these interactions. Concurrent studies include detection of an antigen- binding substance in the hemolymph of infected snails; presence of antigenic mimicry; effects of trematode and snail irradiation or chemical treatment on the host-parasite interaction; and comparative infection and life cycle studies on a series of echinostome parasites collected from various areas endemic for human schistosomiasis. Primary objectives are (1) to characterize and determine the nature of the inter-trematode antagonism; and (2) to search out, identify, and adapt highly predatory trematode species for testing as possible biological control agents against disease-causing trematodes. Collaborating scientists in Malaysia, Egypt, Ethiopia, and Brazil are assisting in this effort. Emphasis for the forthcoming third year of the current grant period will be on the host response to homologous and heterologous reinfection; the study of a synergistic effect by which a primary infection enhances the infection rate of a second one; and further screening of trematodes from endemic areas of human disease for suitable candidates for possible biocontrol testing.